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Changes in B2-B19` and B2-B19 martensitic transformation start temperature(Ms and Ms`) of Ti-Ni-Cu shape memory alloys whose Cu-content is more than 5at% by thermal cyclings have been investigated by means of electrical resistivity measurements and thermal cycling tests under constant load. For thermal cyclings under no applied stress, Ms decreased in the solution treated Ti-45Ni-5Cu alloy, while Ms` kept almost constant in Ti-35Ni-l5Cu and Ti-30Ni-20Cu alloys. Ms in the thermo-mechanically treated Ti-45Ni-5Cu alloy, however, did not change. For thermal cyclings under applied stress of 40 MPa. Ms decreased in the solution treated Ti-45Ni-5Cu alloy, while Ms` increased slightly in the solution treated Ti-35Ni-l5Cu and Ti-30Ni-20Cu alloys. For thermal cyclings under applied stress of 120 MPa, Ms kept almost constant in the solution treated 5at%Cu alloy, while Ms` increased in the solution treated l5at%Cu and 20at%Cu alloys.

keyword : Ti-Ni-Cu shape memory alloys, Thermal cycling effect, Substructural refinement effect, Martensitic transformation temperature, Thermo-mechanical treatment

Effects of Ca addition on the microstructural refinement of the Mg2Si phase in Mg-Si alloy have been investigated by utilizing optical microscope, energy-dispersive spectrometer(EDS). scanning electron microscope(SEM), transmission electron microscope(TEM). X-ray diffraction analysis, and image analyser. As-cast microstructure of the alloys investigated consisted of α-Mg matrix and two types of Mg2Si; i.e.. chinese script type eutectic Mg2Si and polygon type primary Mg2Si. Addition of Ca refined the distribution of Mg2Si by providing heterogeneous nucleation sites for Mg2Si; e.g. mean size of Mg2Si is 18㎛ in Mg-2Si and 9㎛ in Mg-2Si-0.2Ca. These microstructural refinement of Mg2Si could be also obtained by increasing cooling rate. By selected area diffraction pattern, the Mg2Si phase-matrix orientation relationship was established. The orientation relationship between the Mg2Si phase and α-Mg matrix was determined as: [311]α-mg//[114]Mg2Si (011)α-Mg// (311)Mg2Si

keyword : Mg-Al-Si-Ca alloy, Refinement of Mg2Si, Effect of Ca addition, Chinese script type of Mg2Si

The influence of heat treatments on the grain boundary morphology of alloy 690 (Ni-30Cr-9Fe) was studied. Serrated grain boundaries were produced by isothermal heat treatment at 800℃ after solution heat treatment at 1150℃ or by control cooling in the temperature range of 800℃ and 400℃ with a rate of 5℃/min or below. The amplitude and wavelength of serrated grain boundaries increased with longer isothermal heat treatment time or slower cooling rates, but showed maximum value at some extent, respectively. Precipitates on the serrated grain boundaries were revealed as Cr23C6 having semi-coherent cube-to cube relationship -{100}matrix//{100}ppt <100>matrix//<100>ppt -with one adjacent grain.

keyword : Alloy 690, Grain boundary morphology, Serrated grain boundary, Heat treatment, Cooling rate, Cr23C6, Semi-coherent interface, Orientation relationship

The main objective of this study is to investigate the microstructural evolution during mechanical milling of prealloyed (Ti52Al49)95B5 powders and also to investigate the effects of B contents, mechanical milling process and the heat treatment on the microstructure of mechanically milled TiAl alloys. γ-(Ti52Al48)95B5 alloys have been produced by mechanical milling in an attritor mill using prealloyed powders. The grain sizes of the as-milled powders are nanocrystalline. It has been shown that the initial γ phase in as-melt extracted state becomes disordered by mechanical milling. In addition, there is a formation of the amorphous phase with the same composition as the disordered γ phase. Disordered γ and amorphous phases eventually transforms to the nanocrystalline Ti-Al hcp solid solution after the completion of milling. The nature of the final phase depends on the relative stabilities of various phases, modified by the deformation induced by mechanical milling. The mechanically milled (Ti52Al48)95B5 powders are consolidated by vacuum hot pressing at 1000℃ for 2 hours. Microstructure of as-pressed alloys consists of (α2+γ) phase with fine dispersion of TiB2 and TiB phase.

keyword : Microstructural evolution, Mechanical milling, TiAl Alloy, Nanocrystalline, TiB2, TiB, Ti-Al hcp solid solution

The effects of Sn contents on the mechanical properties and the role of Sn in the Cu-9Ni-xSn alloys have been investigated and the following results have been obtained. The mechanical properties such as tensile strength, elongation and electrical resisitivity of Cu-9Ni-xSn alloys are strongly dependent on aging temperature, aging time and Sn contents. With increasing Sn contents, the tensile strength is increased and the temperature where the spinodal decomposition occurs and the incubation time to start spinodal decomposition are lowered and reduced, respectively. By aging treatment at temperatures below 350℃, the 5%Sn alloys. In the case of the 5%
keyword : Cu-Ni-Sn alloys, Spinodal decomposition, Metastable phase, Precipitation, Discontinuous precipitates, Ordered structure, Tensile strength, Elongation, Electrical resistivity

The present study is aimed at developing the high strength steel plates containing tramp elements. Four kinds of Fe-0.2C-l.5Mn-1,5Si-(Cu, Cr, and Ni) hot-rolled steel plates were prepared, and microstructural observation and tensile tests were conducted, The main emphasis has been placed on understanding the effects of tramp elements (Cu, Cr, and Ni) and the coiling temperature (CT) on microstructures and properties of the hot-rolled steel plates. In the case of coiling at 450℃, the higher volume fractions of retained austenite were obtained than the case of coiling at 400℃, and the tensile elongations and the values of strength-elongation balance were increased owing to the transformation induced plasticity (TRIP). The addition of Ni resulted in a great increase of the retained austenite volume fraction and a discontinuous yielding behavior. On the other hand, the addition of Cr or Cr+Ni showed a dual phase stress strain curve having high tensile strength and low elongation.

keyword : Tramp Elements, High Strength Hot-Rolled Steel Plates, Microstructure, Mechanical Property, Retained Austenite, Transformation Induced Plasticity

Formation range of the amorphous phase in melt-spun Al-Ni-Y alloy system was investigated. The amorphous single phase was obtained when the content of solute(Ni+Y) is more than 10at%. Then, the addition of Y atom was more effective than that of Ni atom. The crystallization of amorphous Al90-xNi10Yx, (x=2, 4, 6 at%) alloys proceeded through two or three stages. I.e., by forming Al phase at the first stage of the low temperature side and by forming intermetallic compounds (Al3Ni, Al3Y) at the 2nd or 3rd stages of the high temperature side. The resulting Al particles smaller than l5nm and compounds larger than 50 nm in diameter were uniformly distributed in the amorphous matrix. The ultimate tensile and yield strengths of their alloys kept high value with a slight decrease from room temperature to 400K. Elongation Increased significantly from 2% at room temperature to over 20% at temperatures higher than crystallization temperature. DSC analysis shows that such a large elongation was originated from the precipitation of the crystalline phase. On the other hand, In Al88Ni10Y2 and Al86Ni10Y4 alloys which do not exhibit glass transition phenomenon, the deformation occurred through the shear deformation mode. However, with occurrence of glass transition (Al84Ni10Y6 alloy). the deformation mode was changed viscoelastic alloy.

keyword : Amorphous, High-temperature property, Crystallization, Al particle, Exothermic reaction, Glass transition Phenomenon, Shear deformation, Viscoelastic deformation

A study has been made to investigate the effect of the substitution of tungsten(W) for molybdenum(Mo) on the microstructure and the fatigue crack growth (FCG) behavior of duplex stainless steels. For the homogenized specimens, microstructures and the FCG properties were not largely changed with the Increase in W content. However, when the alloys were aged at 900℃ for 30 minutes after homogenization, the apparent FCG resistance was significantly increased as compared to that of homogenized specimens. It was mainly attributed to the precipitation of the e and x phases in the aged microstructures, which induced higher levels of crack closure than the homogenized microstructures during the crack propagation. With an increase in W content, the quantity and size of the brittle precipitates decreases and the ductility of the aged alloys Increases. Accordingly, the intrinsic FCG resistance of the aged alloys is Improved with increasing W contents.

keyword : Fatigue crack growth, Duplex stainless steel, phase, Crack closure

A study has been made to investigate the microstructural aspects of phase/grain boundary sliding and its accommodation mechanism/mode during superplastic deformation in a two phase Ti-6Al-4V alloy. Tensile tests have been carried out at 600℃ and 900℃ using specimens of 3.2 and 10.6 μm grain sizes. Deformed microstructures have been analysed by transmission electron microscopy to understand the deformation mode, accommodation mechanism, and relative sliding rate of the phase boundary and grain boundaries, In the fine grain (3.2 μm) microstructure deformed at 600℃ and 900℃, dislocations were homogeneously distributed in the α-phase with low density implying that deformation mode follows an iso-stress model. However, the coarse-grain specimens (10.6 μm) reveal the planar slip behavior in the α-phase which indicates that the deformation mode is of a mixed type between iso-stress and iso strain rate mode. It is also considered that accommodation for phase/grain boundary sliding in the α and β phase mainly results from a dislocation activated process, The generation of boundary dislocations is more concentrated along α/β phase boundaries than the α/α grain boundaries, which confirms the sliding resistance of α/β < α/ α.

keyword : Superplastic deformation, Creep, Ti-6Al-4V, Phase/grain boundary sliding, Accommodation mechanism, Accommodation mode, Transmission electron microscopy

The effect of alloying elements on the mechanical properties was investigated to develop the high strength sheet steel for hot dip coating. Silicon and manganese were known as the elements deteriorating the wettability of molten zinc by oxide film formed during annealing. Therefore, phosphorous and titanium were added as solid solution and precipitation hardening element, respectively. The increase of strength by 0.01% addition of alloying element was as follows : 10.3MPa by P. 22.6MPa by Ti+14.7MPa by Ti+Nb. 0.77 MPa by Mn and 7.7MPa by C. An addition of phosphorous and titanium resulted in providing high strength steels and reducing amount of manganese addition.

keyword : High strength steel, Hot dip coating, Rephosphorous Ti-added. (Ti. Nb) (C. N). Precipitates